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Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery grey, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable isotope element to be discovered (rhenium was identified two years later). Hafnium was found by Dirk Coster and George von Hevesy in 1923 in Copenhagen, Denmark, and named Hafnia after the Latin name for "Copenhagen". Hafnium is used in filaments, electrodes, and some semiconductor fabrication processes for integrated circuits at 45 nm and smaller feature lengths. Its large neutron capture cross-section makes hafnium a good material for neutron absorption in control rods in nuclear power plants. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten. Characteristics The physical properties of hafnium metal samples are markedly affected by zirconium impurities, as these two elements are among the most difficult ones to separate because of their chemical similarity. A notable physical difference between them is their density (zirconium being about half as dense as hafnium). The most notable physical property of hafnium is its high thermal neutron-capture cross-section, and the nuclei of several hafnium isotopes can each absorb multiple neutrons. Hafnium does react in air to form a protective film that prevents any further reaction. At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186. The five stable isotopes are in the range of 176 to 180. The radioactive isotopes' half-lives range from only 400 ms for 153Hf, to 2.0 petayears (1015 years) for the most stable one, 174Hf. The nuclear isomer 178m2Hf is also a source of cascades of gamma rays whose energies total 2.45 MeV per decay. It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element. One gram of this pure isotope could release approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers. As a tetravalent transition metal, hafnium forms various inorganic compounds, generally in the oxidation state of +4. The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulphur, and silicon. Due to the lanthanide contraction of the elements in the sixth period, zirconium and hafnium have nearly identical ionic radii. The ionic radius of Zr4+ is 0.79 Ångström and that of Hf4+ is 0.78 Ångström. This similarity results in nearly identical chemical behaviour and in the formation of similar chemical compounds. The chemistry of hafnium is so similar to that of zirconium that a separation on chemical reactions was not possible, only the physical properties of the compounds differ. The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements. Like zirconium, hafnium reacts with halogens forming the tetrahalogen compound with the oxidation state of +4 for hafnium. Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium. The white hafnium oxide (HfO2), with a melting point of 2812 °C and a boiling point of roughly 5100 °C, is very similar to zirconia, but slightly basic. Hafnium carbide is the most refractory binary compound known, with a melting point over 3890 °C, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3310 °C. This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide tantalum hafnium carbide (Ta4HfC5) possesses the highest melting point of any currently known compound, 4215 °C. Value The base value of each unit of ranges between 20 and 50Ð per unit, with up to 3 units being found at any one time. Presence on Mars: Very Rare Category:Chemical elements